The present invention relates to a method and apparatus for treating fiber suspension. The method according to the invention is particularly suitable in screening pulps of the wood processing industry. The apparatus according to the invention relates to a rotor and a screen construction of the power screen to be used.
According to the prior art there are, in principle, two different types of rotor arrangements which both are commonly used and the intention of which, as known, is to maintain the screen surface clean, in other words to prevent the formation of a fiber mat on the screen surface. An example of one type is a rotor arrangement disclosed in the U.S. Pat. No. 4193865 in which a rotor is arranged inside a cylindrical, stationary screen cylinder. The rotor comprises foils located close to the surface of the screen cylinder, which in a construction in accordance with said patent form an angle with the shaft of the cylinder. When moving the foils subject the screen surface to pressure pulses which open the perforations of the surface. There are also arrangements in which the foils are located on both sides of the screen cylinder. Respectively, also the pulp can be fed either to the inside or the outside of the cylinder and the discharge of the accept can take place either from the outside or the inside of the cylinder.
An example of the other type of rotor arrangement is in accordance with the U.S. Pat. No. 3437204, in which the rotor is substantially a cylindrical closed body, on the surface of which there are protrusions almost hemispherical in form. In this kind of apparatus pulp is fed between the rotor cylinder and the screen cylinder outside it, whereby the bulges of the rotor, the so called bumps, act both to press the pulp against the screen cylinder and to draw off the fiber flocks with the trailing edge off the perforations of the screen cylinder. Because this kind of construction has a highly thickening effect on the pulp, there are in the above mentioned arrangement three dilution water connections arranged at different heights on the screen cylinder, so as to make the screening of fiber suspension take place satisfactorily. A corresponding type of a "bump rotor" is disclosed also in the U.S. Pat. No. 3363759, in which the rotor is slightly conical for the reason described further below.
Additionally, other embodiments of the above mentioned cylindrical rotor are known and in connection with which there are intended to be used many kinds of protrusions in the screen cylinder side as disclosed in different publications.
DE application 3006482 discloses a knot separator in which on the surface of a cylindrical rotor drum there are plough like protrusions, made of plate material, by which the pulp between the rotor and the screen cylinder is subjected to strong mixing forces so as to make fibers pass through the screen cylinder most effectively, shives and such separate therefrom.
U.S. Pat. Nos. 4188286 and 4202761 disclose a screen apparatus in which there is a rotable cylindrical rotor inside the screen cylinder. There are protrusions arranged on the rotor on the screen cylinder side, which protrusions have a V-shaped axial cross section such that there is a surface coming closest to the screen cylinder and being parallel to the rim of the rotor, and an end surface substantially perpendicular to the surface of the rotor. These protrusions are arranged on the surface of the rotor cylinder axially in a certain angle position so that all protrusions of the rotor are in the same disposition with respect to the shaft of the rotor.
According to the prior publications pulp can be fed to this apparatus to either side of the screen cylinder. If pulp is fed to the outside of the screen cylinder and accept is discharged from the interior of the screen cylinder, in other words from the rotor side, the rotational direction of the rotor is such that the accept is subjected by the angle position of the protrusions to a force component directed downwards and that the said inclined/ascending surface operates as a front surface. If, however, pulp is fed between the rotor and the screen cylinder, in other words the accept is discharged from exterior of the screen cylinder, the rotational direction is opposite to the former. The protrusions tend to slow down the downward pulp flow and the surface upright to the surface of the rotor cylinder operates as a front surface.
Practical experience in the industry has, however, shown that the above mentioned apparatus arrangements do not operate satisfactorily in all circumstances. For example, the first mentioned foil rotor produces too strong pressure pulses on the accept side of the screen cylinder and is thus not applicable, for example, with the head boxes of paper machines where there are to be no fluctuation of pressure in the suspension. The apparatus also tends to dilute the accept and is therefore not applicable in cases where pulp with constant consistency is needed. Because the foils in the foil rotors are considerably far apart (4 to 8 foils), fiber matting will always form on the screen cylinder before the next foil wipes it off. Thus the use of the screen is not efficient. Additionally, the said rotor type is expensive to produce because of the accurate dimensioning requirements of the rotor and the careful finishing of it.
A substantially cylindrical rotor, described as another model, has protrusions almost hemispherical in form and operates in some circumstances almost ideally, but, for example, in connection with a head box of a paper machine, further claims can be set for its operation. Because the pulp coming to the head box should be of uniform quality in both consistency and in the size of fibers, the power screen should not adversely affect such quality. However, this kind of "bump rotor" tends to dilute the accept and also causes fluctuation in the consistency values. In the performed tests it was noted that a formerly mentioned type of rotor diluted accept in the limits of -0,15 to -0,45% the desired consistency of accept being 3%. Consequently, the consistency ranges, if absolutely calculated, .+-.5% which is too much, when a homogeneous and qualified end product is to be gained. On the other hand, in the screen which comprises a "bump rotor" fractionation also takes place, in other words the mutual relation between the fractions of the fiber suspension fed into the screen cylinder changes in the screen in the way that the relation of the fractions of the accept is no more the same as that of the originally fed pulp. With the "bump rotor" the rate of change of the fractionation has been noted to range between 5 to 10 per cent depending on the clearance between the rotor and the screen cylinder. A corresponding rate of change with the foil rotor was about 20 per cent, thus the bump rotor is already a considerable improvement compared to the earlier apparatuses.
These above described defects of a screen apparatus including a "bump rotor" have led to some attempts at improvement, of which conduction of dilution water to the screen surface and in another case a slightly conical form of the rotor have already been mentioned above. Both methods above reflect a problem arising in connection with a cylindrical rotor, namely unevenness of the screen cylinder use in its different zones. The fact is that the greatest flow through the screen cylinder takes place immediately after the pulp has entered into contact with the cylinder and the rotor. Thus the pulp to some extent thickens and while pulp is flowing down along the screen surface, the amount of suspension passing through the screen perforations reduces constantly. Attempts have been made to prevent this by feeding dilution water at different heights in the screen surface, which results to some extent in a more effective operation of the screen cylinder, but has the drawback of a considerable dilution of the accept. It is also possible to use differing clearance between the screen cylinder and the rotor, whereby a larger clearance of the upper part of the screen apparatus permits a greater downward speed for the pulp with the pulp thus better and more evenly filling said clearance.
A similar manner of operation can also be seen in the arrangement of the U.S. Pat. No. 4188286, in which the protrusions are inclined with respect to the shaft of the screen cylinder. The main purpose of the inclination is to prevent the fibers or fiber flocks from sticking on the front surface of the protrusion and drifting along with it. A secondary purpose is to subject a downward force component to the accept pulp between the rotor and the screen cylinder, which component accelerates the operation of the screen apparatus, or at least the discharge of accept from the screen.
FIG. 1 illustrates typical velocity distribution in a screen apparatus with a cylindrical rotor. The left side of the figure shows the change of axial velocity component V.sub.f of the pulp as a function of the height of the screen cylinder. The right side of the figure, on the other hand, shows the change of velocity component V.sub.z of the suspension flowing through the perforations of the cylinder. The graphs could as well show the change in the volumetric flow, whereby it could be seen that with a conventional arrangement 50 per cent of the accept passes through the perforations of the screen cylinder in the upper quarter of the cylinder and respectively 80 per cent of the accept in the upper half of the cylinder. The theoretical maximum capacity of the screen cylinder is, in use, immediately after the upper edge almost one fifth of the total height of the cylinder. Thereafter the pulp flow which has passed through the cylinder radically reduces due to the radical reduction of the velocity component V.sub.f to less than half of its maximum value in the upper fifth of the cylinder. The reason for this is, of course, both because of the increase of the horizontal velocity component of the pulp due to the effect of the rotor and also thickening of the pulp to some degree between the rotor and the screen cylinder.
Additionally, the right side of the figure shows that only half of the theoretical maximum capacity of the screen cylinder is available for use, while if it were possible to maintain the same velocity through the screen perforations throughout the whole cylinder the graph would be a rectangle and not a curve as in the figure. In reality the capacity is restricted by the amount of reject relatively increasing in the pulp, but only from the middle part of the screen cylinder onwards.
Thus it can be observed that it is possible to increase the capacity of the screen cylinder if the axial velocity of the pulp flowing between the rotor and the screen cylinder can be maintained considerably high and if the pulp can be kept respectively longer in the middle part of the cylinder. FIG. 2 is a graph showing the corresponding distributions as in FIG. 1 for an apparatus in accordance with the invention, whereby it is noted that the axial velocity and respectively also the axial volumetric flow decreases much more slowly than in a conventional arrangement. In other words the velocity V.sub.f has reduced to half of its initial value as late as in the middle part of the screen cylinder. The result of this has been that the screen velocity V.sub.z of the perforations of the screen cylinder has reduced in the upper part of the cylinder due to lesser pressure against the cylinder, but respectively the speed remains constant almost until the middle part of the screen cylinder, wherefrom it evenly reduces but not, however, reducing to zero as in the conventional apparatuses. Thus with this kind of apparatus it is possible to increase the feeding rate, which corresponds the axial velocity V.sub.f, because the maximum screen capacity of the screen cylinder is not yet in use. By such operation the distribution shown in broken lines in FIG. 2 is achieved, which raises the capacity of screen cylinder, roughly speaking, to almost 50% higher.
These results have been achieved by the method in accordance with the invention, characterized in that fiber suspension is additionally subjected to axial forces changing in intensity and effective direction, the direction and intensity of which are determined on the basis of the axial position between the point of application and the counter surface of the screen cylinder and with which the axial speed contour of fiber suspension is changed yet maintaining the flow direction constantly towards the discharge end.
The apparatus according to the invention is, on the other hand, characterized in that at least on one of the said counter surfaces facing another surface there is at least one bulge or corresponding contour or other projection, the direction of the leading or front surface of which varies according to the axial position of the bulge and by which the pulp particle is subjected to an axial force component, the intensity of which varies as a function of the position of the pulp particle in the axial direction, and which changes the speed contour of the fiber suspension flowing between the counter surfaces.
The method and apparatus according to the invention are described in detail below, by way of example with reference to the accompanying drawings, in which: